Another account to mention the ancient language Sanskrit came from Filippo Sassetti (born in Florence in 1540), a merchant who travelled to the Indian subcontinent. Writing in 1585, he noted some word similarities between Sanskrit and Italian (these included devaḥ/dio "God", sarpaḥ/serpe "serpent", sapta/sette "seven", aṣṭa/otto "eight", nava/nove "nine").[6] However, neither Stephens's nor Sassetti's observations led to further scholarly inquiry.[6]

The Ottoman Turkish traveller Evliya Çelebi, who visited Vienna in 1665–1666 as part of a diplomatic mission, noted a few similarities between words in German and Persian. Gaston Coeurdoux and others made observations of the same type. Coeurdoux made a thorough comparison of Sanskrit, Latin and Greek conjugations in the late 1760s to suggest a relationship between them. Similarly, Mikhail Lomonosov compared different language groups of the world including Slavic, Baltic ("Kurlandic"), Iranian ("Medic"), Finnish, Chinese, "Hottentot", and others. He emphatically expressed the antiquity of the linguistic stages accessible to comparative method in the drafts for his Russian Grammar (published 1755).[7]

The hypothesis reappeared in 1786 when Sir William Jones first lectured on the striking similarities between three of the oldest languages known in his time: Latin, Greek, and Sanskrit, to which he tentatively added Gothic, Celtic, and Persian,[8] though his classification contained some inaccuracies and omissions.[9]

It was Thomas Young who in 1813[10] first used the term Indo-European, which became the standard scientific term through the work of Franz Bopp, whose systematic comparison of these and other old languages supported the hypothesis. A synonym for "Indo-European" is Indo-Germanic (Idg. or IdG.), which defines the family by indicating its southeasternmost and northwesternmost branches. In most languages this term is dated or less common, whereas in German it is still the standard scientific term.[11] Advocates of Indo-Germanic often claim that "Indo-European" is misleading because many historic and several living European languages (the unrelated Uralic languages, as well as several others, are also spoken in Europe) do not belong to this family. Advocates of Indo-European counter that Indo-Germanic is misleading because many of the European languages included are not in fact Germanic.

Tocharian, emerged around 3700 BC,[12] extant in two dialects (Turfanian and Kuchean), attested from roughly the 6th to the 9th century AD. Marginalized by the Old TurkicUyghur Khaganate and probably extinct by the 10th century.

Germanic (from Proto-Germanic), emerged around 3300 BC,[12] earliest testimonies in runic inscriptions from around the 2nd century AD, earliest coherent texts in Gothic, 4th century AD. Old English manuscript tradition from about the 8th century AD.

Sicel - an ancient language spoken by the Sicels (Greek Sikeloi, Latin Siculi), one of the three indigenous (i.e. pre-Greek and pre-Punic) tribes of Sicily. Proposed relationship to Latin or proto-Illyrian (Pre-Indo-European) at an earlier stage.[22]

Lusitanian—possibly related to (or part of) Celtic, Ligurian, or Italic

Membership of these languages in the Indo-European language family is determined by genetic relationships, meaning that all members are presumed descendants of a common ancestor, Proto-Indo-European. Membership in the various branches, groups and subgroups or Indo-European is also genetic, but here the defining factors are shared innovations among various languages, suggesting a common ancestor that split off from other Indo-European groups. For example, what makes the Germanic languages a branch of Indo-European is that much of their structure and phonology can so be stated in rules that apply to all of them. Many of their common features are presumed innovations that took place in Proto-Germanic, the source of all the Germanic languages.

To the evolutionary history of a language family, a genetic "tree model" is considered appropriate especially if communities do not remain in effective contact as their languages diverge. Exempted from this concept are shared innovations acquired by borrowing (or other means of convergence), that cannot be considered genetic. In this case the so-called "wave model" applies, featuring borrowings and no clear underlying genetic tree. It has been asserted, for example, that many of the more striking features shared by Italic languages (Latin, Oscan, Umbrian, etc.) might well be areal features. More certainly, very similar-looking alterations in the systems of long vowels in the West Germanic languages greatly postdate any possible notion of a proto-language innovation (and cannot readily be regarded as "areal", either, because English and continental West Germanic were not a linguistic area). In a similar vein, there are many similar innovations in Germanic and Balto-Slavic that are far more likely areal features than traceable to a common proto-language, such as the uniform development of a high vowel (*u in the case of Germanic, *i/u in the case of Baltic and Slavic) before the PIE syllabic resonants *ṛ,* ḷ, *ṃ, *ṇ, unique to these two groups among IE languages, which is in agreement with the wave model. The Balkan sprachbund even features areal convergence among members of very different branches.

Using an extension to the Ringe-Warnow model of language evolution, early IE was confirmed to have featured limited contact between distinct lineages, whereas only the Germanic subfamily exhibited a less treelike behaviour as it acquired some characteristics from neighbours early in its evolution rather than from its direct ancestors. The internal diversification of especially West Germanic is cited to have been radically non-treelike.[23]

Specialists have postulated the existence of such subfamilies (subgroups) as Italo-Celtic, Graeco-Armenian, Graeco-Aryan, and Germanic with Balto-Slavic. The vogue for such subgroups waxes and wanes; Italo-Celtic for example was once a standard subgroup of Indo-European, but is now little honored, in part because much of the evidence it was based on was misinterpreted.[24]

Subgroupings of the Indo-European languages are commonly held to reflect genetic relationships and linguistic change. The generic differentiation of Proto-Indo-European into dialects and languages happened hand in hand with language contact and the spread of innovations over different territories.

Rather than being entirely genetic, the grouping of satem languages is commonly inferred as an innovative change that occurred just once, and subsequently spread over a large cohesive territory or PIE continuum that affected all but the peripheral areas.[25] Kortlandt proposes the ancestors of Balts and Slavs took part in satemization and were then drawn into the western Indo-European sphere.[26]

Shared features of Phrygian and Greek[27] and of Thracian and Armenian[28] group together with the Indo-Iranian family of Indo-European languages.[29] Some fundamental shared features, like the aorist (a verb form denoting action without reference to duration or completion) having the perfect active particle -s fixed to the stem, link this group closer to Anatolian languages[30] and Tocharian. Shared features with Balto-Slavic languages, on the other hand (especially present and preterit formations), might be due to later contacts.[31]

The Indo-Hittite hypothesis proposes the Indo-European language family to consist of two main branches: one represented by the Anatolian languages and another branch encompassing all other Indo-European languages. Features that separate Anatolian from all other branches of Indo-European (such as the gender or the verb system) have been interpreted alternately as archaic debris or as innovations due to prolonged isolation. Points proffered in favour of the Indo-Hittite hypothesis are the (non-universal) Indo-European agricultural terminology in Anatolia[32] and the preservation of laryngeals.[33] However, in general this hypothesis is considered to attribute too much weight to the Anatolian evidence. According to another view the Anatolian subgroup left the Indo-European parent language comparatively late, approximately at the same time as Indo-Iranian and later than the Greek or Armenian divisions. A third view, especially prevalent in the so-called French school of Indo-European studies, holds that extant similarities in non-satem languages in general - including Anatolian - might be due to their peripheral location in the Indo-European language area and early separation, rather than indicating a special ancestral relationship.[34] Hans J. Holm, based on lexical calculations, arrives at a picture roughly replicating the general scholarly opinion and refuting the Indo-Hittite hypothesis.[35]

The division of the Indo-European languages into a Satem vs. a Centum group was devised by Peter von Bradke in his 1890 work, "Concerning Method and Conclusions of Aryan (Indo-Germanic) Studies". In it, von Bradke described a division similar to that of Karl Brugmann's (1886), saying that the original "Aryans" knew two kinds of guttural sounds, the guttural or velar and palatal rows, each of which were aspirated and unaspirated. The velars were to be viewed as gutturals in a "narrow sense," and considered "pure K-sounds." Palatals were "often followed by labialization." This latter distinction led von Bradke to divide the palatal series into a group as a spirant and a pure K-sound, typified by the words satem and centum respectively.[36]

Some linguists propose that Indo-European languages form part of one of several hypothetical macrofamilies. However, these theories remain highly controversial, not being accepted by most linguists in the field. Some of smaller proposed macrofamilies are:

Objections to such groupings are not based on any theoretical claim about the likely historical existence or non-existence of such macrofamilies; it is entirely reasonable to suppose that they might have existed. The serious difficulty lies in identifying the details of actual relationships between language families, because it is very hard to find concrete evidence that transcends chance resemblance, or is not equally likely explained as being due to borrowing (including Wanderwörter, which can travel very long distances). Because the signal-to-noise ratio in historical linguistics declines steadily over time, at great enough time-depths it becomes open to reasonable doubt that it can even be possible to distinguish between signal and noise.

Scheme of Indo-European migrations from ca. 4000 to 1000 BC according to the Kurgan hypothesis.

The proposed Proto-Indo-European language (PIE) is the hypothetical common ancestor of the Indo-European languages, spoken by the Proto-Indo-Europeans. From the 1960s, knowledge of Anatolian became certain enough to establish its relationship to PIE. Using the method of internal reconstruction an earlier stage, called Pre-Proto-Indo-European, has been proposed.

The diversification of the parent language into the attested branches of daughter languages is historically unattested. The timeline of the evolution of the various daughter languages, on the other hand, is mostly undisputed, quite regardless of the question of Indo-European origins.

Using a mathematical analysis borrowed from evolutionary biology, Don Ringe and Wendy Tarnow propose the following evolutionary tree of Indo-European branches:[37]

1500–2000: Early Modern period to present: Colonialism results in the spread of Indo-European languages to every continent, most notably Romance (North, Central and South America, North and Sub-Saharan Africa, West Asia), West Germanic (English in North America, Sub-Saharan Africa, East Asia and Australia; to a lesser extent Dutch and German), and Russian to Central Asia and North Asia.

In reconstructing the history of the Indo-European languages and the form of the Proto-Indo-European language, some languages have been of particular importance. These generally include the ancient Indo-European languages that are both well-attested and documented at an early date, although some languages from later periods are important if they are particularly linguistically conservative (most notably, Lithuanian). Early poetry is of special significance because of the rigid poetic meter normally employed, which makes it possible to reconstruct a number of features (e.g. vowel length) that were either unwritten or corrupted in the process of transmission down to the earliest extant written manuscripts.

Most important of all:

Vedic Sanskrit (c. 1500 – 500 BC). This language is unique in that its source documents were all composed orally, and were passed down through oral tradition (shakha schools) for c. 2,000 years before ever being written down. The oldest documents are all in poetic form; oldest and most important of all is the Rig Veda (c. 1500 BC).

Mycenaean Greek (c. 1450 BC)[40] and Ancient Greek (c. 750 – 400 BC). Mycenaean Greek is the oldest recorded form, but its value is lessened by the limited material, restricted subject matter, and highly ambiguous writing system. More important is Ancient Greek, documented extensively beginning with the two Homeric poems (the Iliad and the Odyssey, c. 750 BC).

Hittite (c. 1700 – 1200 BC). This is the earliest-recorded of all Indo-European languages, and highly divergent from the others due to the early separation of the Anatolian languages from the remainder. It possesses some highly archaic features found only fragmentarily, if at all, in other languages. At the same time, however, it appears to have undergone a large number of early phonological and grammatical changes which, combined with the ambiguities of its writing system, hinder its usefulness somewhat.

Other primary sources:

Latin, attested in a huge amount of poetic and prose material in the Classical period (c. 200 BC – 100 AD) and limited older material from as early as c. 600 BC.

Gothic (the most archaic well-documented Germanic language, c. 350 AD), along with the combined witness of the other old Germanic languages: most importantly, Old English (c. 800 – 1000 AD), Old High German (c. 750 – 1000 AD) and Old Norse (c. 1100 – 1300 AD, with limited earlier sources dating all the way back to c. 200 AD).

Old Avestan (c. 1700 – 1200 BC) and Younger Avestan (c. 900 BC). Documentation is sparse, but nonetheless quite important due to its highly archaic nature.

As in other centum languages, the "plain velar" and "palatal" stops merged, reducing the number of stops from 15 to 12.

As in the other Germanic languages, the Germanic sound shift changed the realization of all stop consonants, with each consonant shifting to a different one:

bʰ → b → p → f

dʰ → d → t → θ

gʰ → g → k → x (Later initial x →h)

gʷʰ → gʷ → kʷ → xʷ (Later initial xʷ →hʷ)

Each original consonant shifted one position to the right. For example, original dʰ became d, while original d became t and original t became θ (written th in English). This is the original source of the English sounds written f, th, h and wh. Examples, comparing English with Latin, where the sounds largely remain unshifted:

Various further changes affected consonants in the middle or end of a word:

The voiced stops resulting from the sound shift were softened to voiced fricatives (or perhaps the sound shift directly generated fricatives in these positions).

Verner's law also turned some of the voiceless fricatives resulting from the sound shift into voiced fricatives or stops. This is why the t in Latin centum ends up as d in hund(red) rather than the expected th.

Most remaining h sounds disappeared, while remaining f and th became voiced. For example, Latin decem ends up as ten with no h in the middle (but note taíhun "ten" in Gothic, an archaic Germanic language). Similarly, the words seven and have have a voiced v (compare Latin septem, capere), while father and mother have a voiced th, although not spelled differently (compare Latin pater, māter).

None of the daughter-language families (except possibly Anatolian, particularly Luvian) reflect the plain velar stops differently from the other two series, and there is even a certain amount of dispute whether this series existed at all in PIE. The major distinction between centum and satem languages corresponds to the outcome of the PIE plain velars:

The three-way PIE distinction between voiceless, voiced and voiced aspirated stops is considered extremely unusual from the perspective of linguistic typology—particularly in the existence of voiced aspirated stops without a corresponding series of voiceless aspirated stops. None of the various daughter-language families continue it unchanged, with numerous "solutions" to the apparently unstable PIE situation:

The Indo-Aryan languages preserve the three series unchanged but have evolved a fourth series of voiceless aspirated consonants.

The Iranian languages probably passed through the same stage, subsequently changing the aspirated stops into fricatives.

The following table presents a comparison of conjugations of the thematicpresent indicative of the verbal root *bʰer- of the English verb to bear and its reflexes in various early attested IE languages and their modern descendants or relatives, showing that all languages had in the early stage an inflectional verb system.

While similarities are still visible between the modern descendants and relatives of these ancient languages, the differences have increased over time. Some IE languages have moved from synthetic verb systems to largely periphrastic systems. The pronouns of periphrastic forms are in brackets when they appear. Some of these verbs have undergone a change in meaning as well.

In Modern Irish beir usually only carries the meaning to bear in the sense of bearing a child; its common meanings are to catch, grab.

The Hindi verb bharnā, the continuation of the Sanskrit verb, can have a variety of meanings, but the most common is "to fill". The forms given in the table, although etymologically derived from the present indicative, now have the meaning of subjunctive. The present indicative is conjugated periphrastically, using a participle (etymologically the Sanskrit present participle bharant-) and an auxiliary: maiṃ bhartā hūṃ, tū bhartā hai, vah bhartā hai, ham bharte haiṃ, tum bharte ho, ve bharte haiṃ (masculine forms).

German is not directly descended from Gothic, but the Gothic forms are a close approximation of what the early West Germanic forms of c. 400 AD would have looked like. The cognate of Germanic beranan (English bear) survives in German only in the compound gebären, meaning "bear (a child)".

The Latin verb ferre is irregular, and not a good representative of a normal thematic verb. In French, other verbs now mean "to carry" and ferre only survives in compounds such as souffrir "to suffer" (from Latin sub- and ferre) and conférer "to confer" (from Latin "con-" and "ferre).

In Modern Greek, phero φέρω (modern transliteration fero) "to bear" is still used but only in specific contexts and is most common in such compounds as αναφέρω, διαφέρω, εισφέρω, εκφέρω, καταφέρω, προφέρω, προαναφέρω, προσφέρω etc. The form that is (very) common today is pherno φέρνω (modern transliteration ferno) meaning "to bring". Additionally, the perfective form of pherno (used for the subjunctive voice and also for the future tense) is also phero.

^In German the scientific term is indogermanisch (Indo-Germanic) indicating the east-west extension. That term first appeared in French as indo-germanique, in 1810, in the writing of Conrad Malte-Brun, a French geographer of Danish descent. In other languages—Dutch, for instance—the general population uses the term Indo-Germaans.

^"The Thracian language". The Linguist List. Retrieved 2008-01-27. An ancient language of Southern Balkans, belonging to the Satem group of Indo-European. This language is the most likely ancestor of modern Albanian (which is also a Satem language), though the evidence is scanty. 1st Millennium BC – 500 AD.

^Fine, John (1985). The ancient Greeks: a critical history. Harvard University Press. p. 72. ISBN 0674033140. "Most scholars now believe that the Sicans and Sicels, as well as the inhabitants of southern Italy, were basically of Illyrian stock superimposed on an aboriginal "Mediterranean" population."

^The supposed autochthony of Hittites, the Indo-Hittite hypothesis and migration of agricultural "Indo-European" societies became intrinsically linked together by C. Renfrew. (Renfrew, C 2001a The Anatolian origins of Proto-Indo-European and the autochthony of the Hittites. In R. Drews ed., Greater Anatolia and the Indo-Hittite language. family: 36-63. Washington, DC: Institute for the Study of Man).

Renfrew, C. (2001). "The Anatolian origins of Proto-Indo-European and the autochthony of the Hittites". In Drews, R. Greater Anatolia and the Indo-Hittite language family. Washington, DC: Institute for the Study of Man. ISBN0-941694-77-1.